8 #include "bytesource.h"
11 // About 99% of all Huffman codes are <= 8 bits long (see codelen.txt),
12 // and it's what libjpeg uses. Thus, it seems like a reasonable size.
13 #define DEHUF_TABLE_BITS 8
14 #define DEHUF_TABLE_SIZE (1 << DEHUF_TABLE_BITS)
15 static const int DEHUF_SLOW_PATH = -1;
17 // About 98% of all AC coefficients (control byte + coefficient) are <= 10 bits
18 // long; again, see codelen.txt. This will cost us about 6 kB of data to store
20 #define DEHUF_AC_TABLE_BITS 10
21 #define DEHUF_AC_TABLE_SIZE (1 << DEHUF_AC_TABLE_BITS)
22 static const int AC_DEHUF_SLOW_PATH = 0xf0000000;
23 static const int AC_END_OF_BLOCK = 0xf0000001;
25 struct huffman_table {
26 unsigned num_codes[17]; // BITS
27 unsigned char codes[256]; // HUFFVAL
30 unsigned huffsize[256];
31 unsigned huffcode[256];
36 // Lookup table for fast decoding; given eight bits,
37 // return the symbol and length in bits. For longer codes,
38 // DEHUF_SLOW_PATH is returned.
40 // Note that the codes we return are 8-bit, but the type of
41 // the lookup tables is int to avoid extra zero extending.
42 int lookup_table_codes[DEHUF_TABLE_SIZE];
43 int lookup_table_length[DEHUF_TABLE_SIZE];
45 // Further lookup tables for decoding AC coefficients.
46 // (Generated but obviously not used for DC coefficients.)
47 // Maps from 10-bit lookahead values to the signed coeffient (_codes),
48 // number of bits to skip (_length) and the number of zero coefficients
49 // after this one (_skip).
50 int ac_table_codes[DEHUF_AC_TABLE_SIZE];
51 uint8_t ac_table_length[DEHUF_AC_TABLE_SIZE];
52 uint8_t ac_table_skip[DEHUF_AC_TABLE_SIZE];
55 enum coefficient_class {
60 typedef struct huffman_table huffman_tables_t[NUM_COEFF_CLASSES][4];
62 // Read Huffman tables from a stream, and compute the derived values.
63 void read_huffman_tables(huffman_tables_t* dst, input_func_t* input_func, void* userdata);
65 unsigned read_huffman_symbol_slow_path(const struct huffman_table* table,
66 struct bit_source* source);
68 static inline unsigned read_huffman_symbol_no_refill(
69 const struct huffman_table* table,
70 struct bit_source* source)
72 assert(source->bits_available >= DEHUF_TABLE_BITS);
73 unsigned lookup = peek_bits(source, DEHUF_TABLE_BITS);
74 int code = table->lookup_table_codes[lookup];
75 int length = table->lookup_table_length[lookup];
77 if (code == DEHUF_SLOW_PATH) {
78 return read_huffman_symbol_slow_path(table, source);
81 read_bits(source, length);
85 static inline unsigned read_huffman_symbol(const struct huffman_table* table,
86 struct bit_source* source)
88 possibly_refill(source, DEHUF_TABLE_BITS);
89 return read_huffman_symbol_no_refill(table, source);
92 // procedure EXTEND (figure F.12)
94 // Fast lookup table for (1 << (bits - 1)).
95 // The table actually helps, since the load can go in parallel with the shift
97 static const int bit_thresholds[16] = {
98 0, 1 << 0, 1 << 1, 1 << 2, 1 << 3, 1 << 4, 1 << 5, 1 << 6, 1 << 7, 1 << 8, 1 << 9, 1 << 10, 1 << 11, 1 << 12, 1 << 13, 1 << 14
101 static inline unsigned extend(int val, unsigned bits)
103 #if defined(__GNUC__) && (defined(__i386__) || defined(__x86_64__))
104 // GCC should ideally be able to figure out that the conditional move is better, but
105 // it doesn't for various reasons, and this is pretty important for speed, so we hardcode.
106 asm("cmp %1, %0 ; cmovl %2, %0"
108 : "g" (bit_thresholds[bits]),
109 "r" (val + (-1 << bits) + 1)
113 if (val < bit_thresholds[bits]) {
114 return val + (-1 << bits) + 1;
121 #endif /* !defined(_DEHUFF_H) */